It is now known to provide satellite-assisted communications between small portable terminals (PT) communicating through a large hub terminal in a star network configuration centered on a network with at least one satellite. One form of this network concept is the Very Small Aperture Terminal (VSAT) network used in commercial systems. Since communications at useable data rates cannot be established directly between two small terminals due to their limited antenna gain-to-temperature ratio (G/T) and effective isotropic radiated power (EIRP), a large hub terminal is required. One such system, as described by H. Goldberg et al. in "A Man-Portable X-Band Terminal System", IEEE MilCom 1989, pp. 457-462, and is shown in FIG. 1; the system 10 has a central satellite 11 and a hub terminal 12 (with an assumed antenna diameter of at least 20') which not only acts as a relay for communications between portable terminals (PTS) 14 (such as the plurality N of PTs, including a first terminal 14a, denominated PT1, up to an n-th terminal 14n, denominated PTn), but also serves as the network control terminal (NCT) controlling access (both of the individual terminals, via links 16a-16n, and the NCT via link 16r) to the limited satellite resource on a demand assignment basis. The demand-assigned multiple access (DAMA) protocol uses (FIG. 2) multiple FDMA channels (within the same satellite transponder) with a single channel 18 for reservation, control and data transmission and a number of separate channels 20a, 20b, 20c, . . . for voice transmissions. Channel time is divided into fixed intervals called frames. Each frame is further divided into four segments: a forward orderwire (FOW) segment 18a; a ranging segment 18b; a return orderwire (ROW) segment 18c; and a data transmission segment 18d. Each of these four segments is divided into a plurality of slots 20. Each time slot 20 accommodates a single burst of information from a single transmit terminal 14. Data transmission occurs in each of the time-sequential flames on channel 18; the M-th frame includes portions 18a-18c for channel reservation purposes and a portion 18d for data communications transmission. The portable terminals will operate in a half duplex mode with data communications provided at rates of between 75 bps and 2.4 kbps and voice communications at a data rate of 2.4 kbps. The network control equipment would be implemented as a rack mounted applique which would be installed in an available fixed-site terminal 12 of adequate size.
The slots 20 of the ranging segment 18c are used by each PT 14 during initial network entry to correct for the relative difference in propagation delay between the hub 12 and the satellite 11, and between the PT 14 and the satellite. Since the PT cannot "hear" its own ranging signal directly, the hub 11 is used to aid in the ranging process. After acquiring the FOW and frame synchronization in the downlink, the PT attempts to transmit its ranging signal such that it arrives at the hub terminal midway through a randomly-chosen slot 20 of the ranging segment 18c. This transmit time is selected based on a predetermined estimate of range to the satellite. The hub terminal, which has performed a self-loop ranging procedure, compares the actual time of arrival of the PT ranging signal to the true midpoint of the ranging slot. It then relays this value to the PT which adjusts its transmit time. The terminal is now frame synchronized in both the downlink and uplink. A similar aiding process is envisioned to resolve doppler and oscillator induced frequency offsets between the PT and the hub. Maintenance of time and frequency synchronization could be performed by periodically accessing a ranging slot or by measuring the offsets of routine PT transmissions such as the ROW signals. Other synchronization techniques which forego the need for ranging such as those which make use of satellite ephemeris data could also be implemented.
Channel resource control is effected through the use of the two orderwires. Terminals request channel access via the ROW and the hub responds to the requests via the FOW. The FOW contains DAMA control information such as channel assignments, system status, access restriction and service preemption. It also contains timing, frequency and cryptographic synchronization information which is needed for terminals to acquire the downlink. The ROW slots are used by portable terminals to request access, acknowledge messages and report status to the hub.
Many techniques for accessing a satellite network by multiple users are available and are well known to those skilled in the satellite communications arts. The choice of a particular channel access protocol is very dependent on the particular application; the low weight and compact size requirements of man-portable terminals present a unique set of design issues to be addressed. It is highly desirable to provide system characteristics and design goals which provide better channel access with a system supporting half-duplex data and voice communications, and a plurality of data transmission rates. Due to power constraints, portable terminals are not capable of communicating directly with each other and all communications have to be rebroadcast by a repeater. Therefore, any desired system will use a minimal number of channels in order to minimize hardware requirements; the system should dynamically assign resources to accommodate voice and data traffic to minimize the average waiting times of traffic and the blocking probability of voice calls.